The invention relates to a method for forming electrically conductive circuits on a base board, and more particularly relates to a method for forming at least four laminations of circuits on the base board, wherein a newly developed electrically conductive copper paste, which is specifically adapted to metal plating, is effectively utilized to form so many laminations of circuits on a single base board, the so many laminations of circuits including a resistor circuit by way of a resistor paste or an electricity storing circuit of a dielectric paste. The finished print-circuit board is extremely thin to the merit thereof.
So far it has been general to form a resistor circuit or an electricity storing circuit on a copper laminated base board by soldering a lead or a chip-shaped resistor or a condenser onto the copper laminated base board. The finished product is therefore bulky in addition to requiring so many precessing steps and the resultant high cost including the cost of the resistor or a condenser. Further according to the conventional method, the loading density of the print-circuit base board is lower, and the reduction of product weight and of production process is difficult. Moreover since the soldering operation is required, there have often been a misarrangement of leads and a misinsertion of the resistor or the condenser.
Further in case of forming considerably complex circuits on the copper laminated base board, it becomes necessary to electrically connect the circuits to each other. According to the prior art, a through-hole is provided to electrically connect the circuits on both sides of the base board because it has been impossible to form the circuits of more than two laminations on one side of the base board. Even if the through-hole is provided, it has been impossible to form more than two laminations of circuits, that is, one located on one side of the base board and the other located on the other thereof.
It has been proposed to form more than two laminations of circuits on one side of a ceramics base board. For example, in case of the hybrid IC, it is general to employ a paste of precious metal such as platinum-palladium or silver-palladium to form the circuits and the terminals thereof, and to employ a paste of ruthenium oxide to form a resistor and then to burn the base board at a high temperature (700.degree. C.-1000.degree. C.). Further it has been proposed to form more than two laminations of circuits on one side of a base board by printing a tungsten (W paste) and an isolating paste alternately on an alumina green sheet and then by burning the alumina green sheet at a temperature of about 1600.degree. C. However according to such methods requiring a high temperature burning treatment, the ingredients of circuit structure to be used are limited and the equipment to be required is costly. The methods are therefore not suitable to produce the print-circuit boards to be generally used in combination with the electronic apparatuses.
It is therefore desired to industrially set up a method for forming more than two laminations of circuits on one side of a pol.ymer base board which may be treated at a lower temperature. In this case, it becomes necessary to develop on electrically conductive copper paste which has a property of electrically high conductivity and is specifically adapted to a metal plating, especially to a copper plating and which may be obtained at a lower cost. It has however been difficult to practically use the electrically conductive paste, because the copper perticles in the paste may be easily oxidized when the paste is heated to be hardened at a temperature of approximately 150.degree., and the paste will have a high electric resistance and reduce the soldering property. Generally the conventional electrically conductive copper paste is easily oxidized with a heat for hardening the paste, in contrast to the precious metal such as silver. The oxidization of copper powder in the paste will increase the electric resistance and decrease the soldering property. These defects have made the conventional electrically conductive paste practically useless. Further it has been required to activate the surface of the hardened electrically conductive copper paste by means of a catalyst so as to expose the copper powder from the resin paste so that the exposed copper powder may act as the binder, that is, so many neucleuses for the subsequent metal plating. Thus the conventional electrically conductive paste has required so many processing steps.
The Japanese utility model application No. 50-932 (publication No. 55-42460) discloses a specific method, in which a high dielectric resist of polybutadiene is used as a dilectric coat, an adhesive paste, for example, of 20% of phenol resin, 63% of copper powder and 17% of solvent is used to form designed circuits, the adhesive paste is thickened up to 20 .mu. by means of a non-electrolytic plating, and then the plated adhesive paste is coated with copper so as to form the electrically conductive circuits of more than two layers on one side of a base board. The mentioned specific method has never been industrially reduced to practice.
The applicant has been engaged in studying new electrically conductive copper pastes for many years to eliminate the defects of the prior art as mentioned above and succeeded to provide such new electrically conductive copper pastes to be industrially employed. The newly developed electrically conductive copper pastes includes the electrically conductive copper paste ACP-020, ACP-030 and ACP-007P of Asahi Chemical Research Laboratory Co., Ltd.. The electrically conductive copper paste ACP-020 is substantially composed of 80% by weight of copper powder and 20% by weight of synthetic resin, and is extremely excellent in the electric conductivity, but more or less deteriorated in the soldering property. The electrically conductive copper paste ACP-030 is substantially composed of 85% by weight of copper powder and 15% by weight of synthetic resin, and is slightly lower than the ACP-020 as to the electric conductivity, but excellent in the soldering property. In the last place, the electrically conductive copper paste ACP-007P is an improvement of the ACP-030 and may be subjected to a metal plating such as a copper chemical plating without using a catalyst. Namely the copper paste is extremely excellent in the metal plating property.